1. Field of the Invention
This invention relates to the fabrication of semiconductor ZnO (zinc oxide) for application in electric and optoelectric devices. Particularly, this invention provides a simple and reproducible process to dope ZnO to make this material have a p-type conductivity. This invention is also related to the process of pulsed laser deposition of transparent thin films, particularly to the deposition of large area transparent thin films and multilayer periodic dielectric structures on transparent substrates.
2. Description of the Related Art
ZnO is a semiconductor material with a large direct band-gap of 3.37 eV at room temperature. Due to this large band-gap and a large exciton binding energy (60 meV), ZnO has great potential for use in short-wavelength optoelectronic devices, such as light-emitting diodes (LEDs), laser diodes (LDs), and ultraviolet light detectors. In the past years, this field has been dominated by other materials such as GaN and SiC, which are very expensive to fabricate. In comparison, the cost of making ZnO is very low. For this reason, ZnO has also been considered for large scale applications such as solid-state lighting, transparent electronics, flat-panel displays, and solar cells. However, ZnO is intrinsically n-type, and the unavailability of a reliable process to fabricate robust p-type ZnO is the bottleneck in the commercialization of ZnO-based devices.
Previously, nitrogen (N) doping has been the most widely used method to produce p-type ZnO. However, use of this method involves a compromise between nitrogen solubility and film structural quality. This is because high structural quality requires high growth temperatures, whereas the nitrogen solubility decreases with growth temperature. WIPO publication WO0022202 provides a Ga and N co-doping approach to achieve p-type conductivity in ZnO with a high N concentration at the substitutional sites for oxygen (and therefore a high hole concentration). However, the results of a few other attempts (K. Nakahara et al., Journal of Crystal Growth, Vol. 237-239, 503, 2002; M. Sumiya et al., Applied Surface Science, Vol. 223, 206, 2004) using this co-doping method are inconsistent and irreproducible. More recently, a process called ‘temperature-modulated growth’ was disclosed in WIPO publication WO05076341. This method deals with the mutual exclusivity between N solubility and film structural quality by periodically fast ramping the growth temperature, which, in practice, is a very complicated process discloses a method of fabricating p-type ZnO film by co-doping N with alkali metal elements. It is believed that the co-doping with the alkali metal atoms leads to the compensation of donor defects in ZnO film and eventually enhances the p-type conductivity.
In the above N doping approaches, either gas sources, such as NO and NO2 (U.S. Pat. No. 6,908,782) or plasma sources that discharge N2, N2O, NO or NO2 gases are employed. However, using nitrogen oxide (NOx) gasses inevitably results in negative environmental impacts. In addition, there are technical shortcomings in N doping, which are addressed in, e.g., E. C. Lee et al., Phys. Rev. B Vol. 64, 085120, 2001; and H. Matsui, et al., J. Appl. Phys. Vol. 95, 5882, 2004. For example, nitrogen-related donor defects can be generated in the doping process due to the competition between electron-impact and gas-phase reactions, which often occur within the plasma source and during the growth.
In addition to nitrogen, other group-V elements, such as phosphorus (P) and arsenic (As) have also been used as alternative dopants (K. K. Kim, et al., Appl. Phys. Lett. 83, 63, 2003; Y. R. Ryu, et al., Appl. Phys. Lett. 83, 87, 2003; D.C. Look et al., Appl. Phys. Lett. 85, 5269, 2004; U.S. Pat. No. 6,610,141). However the reported results have not been widely confirmed.
The invention uses pulsed laser deposition to grow the ZnO films and films of other materials. Pulsed laser deposition (PLD) is a powerful tool for growth of complex compound thin films. In conventional nanosecond PLD, a beam of pulsed laser light with a typical pulse duration of a few nanoseconds is focused on a solid target. Due to the high peak power density of the pulsed laser, the irradiated material is quickly heated to above its melting point, and the evaporated materials are ejected from the target surface into a vacuum in a form of plasma (also called a plume). For a compound target, the plume contains highly energetic and excited ions and neutral radicals of both the cations and the anions with a stoichiometric ratio similar to that of the target. This provides one of the most unique advantages of PLD over the conventional thin film growth techniques such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). The characteristics of this growth method have been reviewed in several recent journal articles and are summarized in the book by Chrisey and Hubler. See, e.g., P. R. Willmott and J. R. Huber, Pulsed Laser Vaporization and Deposition, Review of Modern Physics, Vol. 72 (2000), pp 315-327; J. Shen, Zhen Gai, and J. Kirschner, Growth and Magnetism of Metallic Thin Films and Multilayers by Pulsed Laser Deposition, Surface Science Reports, Vol. 52 (2004), pp 163-218; and D. B. Chrisey and G. K. Hubler, Pulsed Laser Deposition of Thin Films, John Wiley & Sons, Inc., New York, 1994.
With the appearance of commercially available ultra-fast pulsed lasers (with typical pulse durations of a few picoseconds down to tens of femtoseconds), ultra-fast PLD has attracted much attention. First, due to the extremely short pulse duration and the resultant high peak power density, multiphoton excitation of free carriers becomes significant in transparent materials, and the critical fluence of ablation can be reduced by 1-2 orders of magnitude compared with conventional nanosecond laser ablation. As a result, the commonly favored ultraviolet wavelength in nanosecond laser ablation is no longer a requirement in ultra-fast PLD. In fact, focused ultra-fast pulsed infrared lasers have been successfully used to ablate wide band gap materials. Second, when the laser pulse duration is shorter than the time scale of carrier-phonon interaction (typically a few picoseconds), heat diffusion in the target is negligible. For this reason, ultra-fast PLD has been considered as a good solution to the problem of droplet generation that has long been hindering wider application of PLD. Another advantageous result of the limited heat diffusion is that the removal of target materials is confined to the area within the laser focal spot. This mechanism has enabled the precise laser machining with submicron resolution using ultra-fast lasers.
With the advantages of PLD, especially those related to the ultra-fast PLD, in this invention, we also consider the application of PLD in the field of direct deposition of patterned structures. There have been several types of direct writing techniques involving application of pulsed lasers. (Here ‘writing’ means either adding materials onto substrates, i.e., deposition, or removing materials from substrates, i.e., etching.) For writing patterned materials by means of deposition, laser chemical vapor deposition (LCVD) utilizes laser-enhanced decomposition of CVD precursors for metals, and can be used for depositing metal lines and dots. Another technique is the laser-induced forward transfer (LIFT). See J. Bohandy, B. F. Kim, and F. J. Adrian, Metal Deposition from a Supported Metal Film Using an Excimer Laser, Journal of Applied Physics, Vol. 60 (1986), pp 1538-1539.
In LIFT, a metal thin film is first coated on one side of a transparent target substrate. A pulsed laser beam is incident from the other side (i.e., the uncoated side) of the target substrate and is focused on the front side (i.e., the coated side). The laser ablates the metal film and transports the metal vapor to the surface of a receiving substrate, which is positioned very close to the target substrate (10 μm or less). Various forms of LIFT have been proposed and are described in several U.S. patents cited or referenced by this application.
There are a few limitations in the application of the above two techniques. LCVD involves a complex CVD system and toxic metalorganic gases. In LIFT, the thin metal film limits the amount of material that can be deposited. Also, because the metal thin film is supported by a target substrate, ablation of the target substrate surface that is in direct contact with the film can be involved, which contaminates the deposited material. Finally, both techniques are suitable only for deposition of metals.
In order to transfer other types of material, there is known a variation of LIFT, matrix-assisted pulsed laser evaporation (MAPLE) and direct write, in which the material to be transferred is mixed with a matrix material which is volatile and easy to be ablated and pumped away. See P. K. Wu et al., Laser Transfer of Biomaterials: Matrix-Assisted Pulsed Laser Evaporation (MAPLE) and MAPLE Direct Write, Journal of Applied Physics, Vol. 74 (2003), pp 2546-2557. The mixture is then coated on the supporting target substrate as in the LIFT method. The MAPLE method is suitable for transferring bio-materials without destroying the biomolecules. For dielectrics, the deposits often remain in their original powder form, and adhesion and purity can be problematic due to the non-epitaxial nature and contamination by the matrix material, respectively.
Other laser-assisted direct depositing techniques include laser ink jet printing and Micropen© techniques. Both are wet techniques (i.e., involving liquid binders) and are not suitable for electronic and photonic applications. Therefore, for direct deposition of patterned high purity dielectric materials, in particular, by the way of growth (e.g., epitaxy), a suitable method is still lacking.